1. Field of Invention:
This invention relates in general to power converters. More specifically, the invention relates to power converters of the type used for driving an AC motor. The invention provides a power converter that provides a stable drive for the AC motor even when there is instantaneous power failure of an input AC power source of the converter. The term "instantaneous power failure" is meant to include an instantaneous voltage drop.
2.Description of the Prior Art
Power converters of various types are known. The present invention pertains to a power converter type generally referred to as a voltage inverter. The invention will be described using an induction motor as an example of a conventional AC motors to which AC power is supplied from the power converter according to the present invention. However, the invention is not to be considered as being limited to this specific example. It is only so described as a matter of convenience.
FIG. 1 (Prior Art) shows a main circuit configuration of a conventional voltage type inverter. AC power of an input AC power source 11 is converted to DC power by a forward conversion circuit 12, also referred to as a rectifier circuit. Ripple components of converted DC power are eliminated by a DC reactor 13 and a smoothing capacitor 14. Smoothed DC power is converted into AC power, which generally differs from the original AC power from source 11 in frequency and voltage.
AC conversion is carried out by an inverter circuit 15 which provides power to drive an induction motor 16. Inverter circuit 15 includes a plurality of a gate turn-off (GTO) devices. The detailed operation of such GTO devices is described in "Principle and Applications of Gate turn-off Thyristors" by Nagataka Seki, published by Denki Shoin. The description of circuit 15 per se is omitted, because it is not particularly significant for appreciation of the present invention and within the abilities of one of ordinary skill in the art to practice.
FIG. 2 (Prior Art) is a diagram illustrating the main circuit of FIG. 1 (Prior Art) and the control circuit thereof. A voltage controlled oscillator (VCO) 22, connected to a frequency reference setter 21, produces pulses with a frequency of six (6) times the inverter output frequency f in accordance with a frequency reference established by the frequency reference setter. An output of voltage controlled oscillator 22 is divided into the reference frequency f by a frequency divider 23, which in turn supplies an output to inverter circuit 15.
The frequency reference fr is also coupled, as a voltage reference, to a comparator 24 for comparison with a voltage feedback signal derived from a voltage detection circuit 25 provided with a transformer that detects the inverter output voltage and a rectifier that rectifies the thus detected voltage. The output from comparator 24 is coupled to a voltage control circuit 26 in which the thus compared result, i.e., an error voltage is amplified so as to be coupled as a phase reference into a phase control circuit 28. Phase control circuit 28 produces gate pulses according to the phase reference in synchronism with a phase signal derived from a transformer 27 that detects a voltage phase of the AC power source 11, so as to feed the same to the respective thyristor gates that constitute the rectifier circuit 12.
An instantaneous interruption detector 29 connected to the transformer 27 detects an instantaneous interruption of the AC power source 11 and judges that a power failure has occurred when the instantaneous interruption continues longer than a certain specified time. It then provides a signal to frequency divider 23 and phase control circuit 28 to stop operation of the inverter. The operations of the voltage type inverter shown in FIG. 2 (Prior Art) upon occurrence of instantaneous interruption will be described with reference to FIG. 3 (Prior Art) and FIG. 4 (Prior Art).
In FIG. 3, line (a) represents an instantaneous interruption signal e.sub.a detected by instantaneous detector 29 line (b) represents a charging voltage Ec of smoothing capacitor 14. Line (c) represents an inverter output current Io which is an AC current that flows into the induction motor from the inverter circuit 15 represented by a DC level, respectively.
FIG. 4 (Prior Art) is a diagram illustrating speed - torque characteristics of the induction motor 16, and the ordinate thereof represents a torque T, and the abscissa a speed N, respectively.
In FIG. 4 (Prior Art), the curve Qa represents a speed-torque characteristic in the case of normal operation, the curve Qb a speed - torque characteristic in the state of instantaneous interruption. Curve Q.sub.L represents a load - torque characteristic of induction motor 16, respectively. Assuming that when the induction motor 16 rotates at a speed of n.sub.1 at a time t.sub.1 should an instantaneous interruption occur, the charging voltage E.sub.c begins to decrease from time t.sub.1. In this case, should a control be performed such that the inverter output frequency f corresponding to a synchronous speed m.sub.o is maintained constant, the inverter output current I.sub.o increases in order to obtain the same output level as before the instantaneous interruption, so that the charging voltage Ec decreases more significantly. At a time t.sub.2, the induction motor 16 rotates with the speed - torque characteristic shown in curve Qb. After the speed of induction motor 16 reaches a speed n.sub.2 (corresponds to the maximum torque), the induction motor loses speed, and its speed decreases in a free-run state.
Should the power recover at a time t.sub.3 which is during the free-run state, a rush current appears and the smoothing capacitor 14 is overcharged because of the charging voltage. In this case, Ec of the smoothing capacitor 14 has been considerably reduced.
In addition to these disadvantages, there is another disadvantage. The ratio of voltage and frequency (hereinafter, referred to as v/f) of the induction motor 16 is reduced because of its lost speed. The flux thereof is not sufficiently established, because of its lost speed, when an attempt is made to operate with the v/f ratio previous to the instantaneous interruption. Therefore a rush current appears, resulting that both the main circuit and the control circuit become unstable. In addition to the aforementioned control system, the invention provides an arrangement whereby upon detection of an instantaneous interruption, the inverter output frequency is instantaneously reduced so as to establish a regenerative state whereby the charging voltage Ec of the smoothing capacitor 14 and the flux of the induction motor 16 are ensured. However, there still exists a disadvantage that the speed of the induction motor 16 reduces more rapidly than the natural deceleration during the period of instantaneous interruption.
Moreover, there has been provided a control circuit of FIG. 5 (Prior Art) that can continue a stable drive of an AC motor upon occurrence of an instantaneous interruption. In FIG. 5 (Prior Art), a speed reference wm* established by a setter 31 is fed into a comparator 32 so as to be compared with a speed feedback signal wm from a speed detector 16'. A speed control circuit 34 receives the thus compared result and amplifies the same so as to produce a torque current component comment i.sub.1 q*. A flux reference .phi.* established by a setter 35 is fed into a comparator 36 so as to be compared with a flux feedback signal .phi.. The flux feedback signal .phi. is produced as a result of a calculation performed within a flux calculation circuit 39, which receives a terminal voltage V.sub.1 of the induction motor 16 derived through a potential transformer 37 and a terminal current i.sub.1 of the induction motor 16 derived through a current transformer 38 so as to perform the calculation. A flux control circuit 40 receives the compared result from the comparator 36, which is an error signal, and amplifies the same so as to produce an exciting current component command i.sub.1 d*.
A current command calculation circuit 41 performs a calculations of EQU .vertline.i.sub.1 o*.vertline.=.sqroot.i.sub.1 q*.sup.2 +i.sub.1 d*.sup.2 EQU .angle.i.sub.1 o*=tan.sup.-1 (i.sub.1 q*/i.sub.1 d*)
so as to produce a current command i.sub.1 0*. In addition, a vector rotation device 42 performs a calculation of EQU .vertline.i.sub.1 *.vertline.=.vertline.i.sub.1 o*.vertline. EQU .angle.i.sub.1 *=.angle.i.sub.1 o*+.angle.U.phi.
so as to produce a AC current command i.sub.1 *. Here, the vector rotation device 42 receives a flux phase U.phi. from the above-described flux calculation circuit 39. A comparator 43 receives the AC current command i.sub.1 * so as to be compared with an actual current signal i.sub.1. The thus compared result is fed into a PWM (pulse width modulation) control circuit 44 so as to produce ON-OFF signals which, in turn, are fed into the respective GTO thyristors within the inverter 15. On the other hand, an instantaneous interruption detection circuit 46 receives a voltage signal of the AC power source 11 through a potential transformer 45 so as to detect whether the received voltage signal exists above or below a specified instantaneous interruption level, and when detected as an instantaneous interruption, then produces an instantaneous interruption signal e.sub.a.
In general, the control circuit of FIG. 5 has not sufficiently utilized the instantaneous interruption signal e.sub.a. Namely, upon occurrence of instantaneous interruption, it is uncertain how long the operation of the system can continue by the accumulated charge on the smoothing capacitor 14, so that when the period of an instantaneous interruption exceeds a specified period, the operation of the system is usually stopped. This is also because of the configuration of the control circuit in FIG. 5 which functions in such a manner of so-called vector control based on an instantaneous value control, and should an attempt be made to successfully achieve a vector operation even upon occurrence of an instantaneous interruption, the control circuit would be extremely complicated. This has been another disadvantage hitherto.